COMPBRN3 is a deterministic fire hazards program designed for use in
probabilistic analysis of fire growth in a particular room. This analysis is required when
assessing the risk associated with fires in nuclear power plants. An air entrainment model
is included which consists of a plume entrainment model, a wall jet entrainment model,
and a model for the doorway mixing effect. Output available includes the total heat
release rate of the fire, the temperature and thickness of the hot gas layer formed near the
compartment ceiling, the mass burning rate for individual fuel elements, the surface
temperature of the elements, and the thermal heat flux at user-specified locations.

5. METHOD OF SOLUTION

A quasi-static approach is followed to simulate the process of fire growth during
the pre-flashover period in an enclosure. The compartment is modeled as two zones (or
control volumes) which divide it into two distinct, homogeneous, stably-stratified
regions. The hot gases accumulating under the ceiling due to fire plume entrainment and
negative buoyancy are defined as the upper layer (the ceiling hot gas layer). The lower
region is assumed to be thermally inert and contains relatively quiescent cool air, which
remains at ambient conditions at all times. These regions are separated by a thermal
interface with uniform height inside the room and a higher elevation at the doorway. At
each time-step, the hot gas layer model computes the thermal interface heights, the hot
gas layer temperature, and the heat fluxes to fuel elements by solving several coupled
mass and heat steady-state balance equations by a Newton-Raphson iteration scheme.
Correlations are used to determine the convective heat transfer in the buoyant plume of
hot gases above the flames. The COMPBRN3 steady-state models and submodels are
taken from the fire research literature.